Savings and Deferred Compensation Plans Savings and Deferred Compensation Plans (Savings Plans) (Details) - USD ($) $ in Thousands	12 Months Ended
Oct. 02, 2020	Sep. 27, 2019	Sep. 28, 2018
Savings And Deferred Compensation Plans [Abstract]
Savings plans contributions	$ 91,833	$ 114,006	$ 113,135

Defined Contribution Plan Savings Plans Contributions

No definition available.

Name:	jec_DefinedContributionPlanSavingsPlansContributions
Namespace Prefix:	jec_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration